1. Field of the Invention
The present invention relates to a light-depreciation type smoke detector and, more particularly, to a smoke detector which is not sensitive to the depreciation of light attributable to secular change, or a change of ambient temperature, but can produce an alarm upon detection of an average smoldering fire, within an acceptable time.
2. Description of the Prior Art
A known typical conventional light-depreciation type smoke detector is disclosed in U.S. Pat. No. 3,846,772.
Such detector incorporates a comparator adapted to produce an output when the voltage differential between a first and a second electric signals, which vary depending on the amount of received light and are input to the comparator at a suitable interval, comes to exceed a predetermined threshold voltage. In order that the second input signal may suitably be compared with the first input signal, a capacitor for holding the voltage of the first input signal, and another capacitor for holding the voltage of the second input signals are connected to respective terminals of the comparator.
The output from the comparator is obtainable only when a reduction in the amount of the received light corresponding to the above mentioned threshold voltage for the functioning of the comparator takes place in the period from the time of receipt of the first input signal to the time of receipt of the second input signal. However, in case of a smoldering fire in which the reduction of the amount of light takes place in quite a slow and gentle manner, a change of the voltage of the first input signal tends to occur, making it difficult to correctly make use of the predetermined voltage differential between the first and the second inputs.